PROJECT SUMMARY Loss of muscle mass during periods of disuse due to illness or hospitalization and poor recovery have serious health consequences for older adults. The mechanisms underlying the atrophy response to disuse and subsequent recovery in humans, however, are not known. We posit that poor mitochondrial energetics and altered muscle lipid metabolism contribute to greater disuse- induced muscle atrophy and poor recovery. In addition, we believe that older subjects with type 2 diabetes have reduced mitochondria capacity and excess muscle lipids, which exacerbates the deleterious effect of bed rest on muscle mass and impairs recovery. Mitochondria play a fundamental role in muscle health and impaired mitochondrial energetics and muscle lipids have been shown to contribute to human aging and type 2 diabetes. The role of mitochondrial energetics in human muscle atrophy and recovery is not known. In this project, we will test the hypothesis that altered mitochondrial energetics in muscle atrophy and recovery via cardiolipin remodeling, elevated reactive oxygen species (ROS) emission and accumulation of specific muscle sphingolipids blunt anabolic stimuli and contribute to muscle atrophy and poor recovery. The overall objective of this project is to decipher mechanisms by which mitochondrial energetics and muscle lipids underlie early muscle catabolic and anabolic responses to disuse and recovery. Studying older adults with type 2 diabetes and using an exercise recovery program will provide a physiological context and will allow us to delineate novel mechanisms. Through innovative time course studies and measurements in serial human muscle biopsies, we will place the etiology of muscle atrophy firmly in the context of mitochondrial biology and will contribute to a better precision medicine approach to prevent and treat disuse atrophy.